1. Field of the Invention
The present invention relates to dispersion hardening of copper and, in particular, to a process for producing dispersion hardened copper alloys and use of such alloys for spot welding electrodes.
2. Description of the Related Art
Presently known processes for dispersion hardening of copper are of three types. Extremely fine, and thus very expensive, powders of copper matrix metal have been carefully mixed with a dispersoid, such as extremely fine particles of Al.sub.2 O.sub.3 or BeO and compacted and extruded. Alternately, alloys of copper matrix metal have been prepared and contain small percentages of easily oxidized metals, such as beryllium or aluminium. These prepared alloys are then processed into powders and the powders are subjected to internal oxidation, an expensive step which, if controlled correctly, results in the desired fine distribution of oxide particles in the matrix, with the oxide particles having a diameter of less than 0.1 .mu.m. This process, however, has the drawback that copper is also externally oxidized during the internal oxidation step. This necessitates a further heat treatment with hydrogen in order to reduce copper oxide to copper. During this further heat treatment, however, it is almost impossible to avoid baking together of the powders. Moreover, both of the aforementioned prior art processes are expensive and, thus, have not been widely accepted. Simultaneous precipitation of copper matrix metal and dispersoid from appropriate metal salt solutions is also known, but is too expensive for industrial use.
Generally, all metals including copper dispersion hardened with oxides of the aforementioned type exhibit great brittleness when heated to around about 500.degree. C. and above. Further, ductility decreases with increasing temperature and reaches a minimum of about 2% at about 500.degree. C. compared to the good ductility at room temperature for which elongation values around about 20% are typical. Embrittlement and loss of ductility constitute grave drawbacks of prior art dispersion hardened alloys.